Publications

AbstractLarge-scale atomistic simulations are performed to study different mechanisms
of plastic deformation in uncapped submicron thin polycrystalline copper films.
It was recently shown that diffusional mass transport along grain boundaries
in thin films leads to the formation of a novel defect identified as a diffusion
wedge (Gao et al 1999 Acta Mater. 47 2865–78). Eventually, a crack-like stress
field develops near the grain boundary–substrate junction as tractions along the
grain boundaries relax under the constraint that the adhesion between film and
substrate prohibits strain relaxation close to the interface. The emergence of
crack-like stress concentration causes nucleation of an unexpected class of
dislocations near the root of the grain boundary on glide planes parallel to the
film surface. These dislocations are unexpected because there is no driving
force for parallel glide (PG) in the overall biaxial stress field. In this work,
we demonstrate that PG dislocations dominate plasticity in polycrystalline
submicron thin films when tractions along the grain boundaries are relaxed by
diffusional creep. We illustrate that partial dislocations play an important role
in plasticity of nanostructured thin films and that the grain boundary structure
has a significant influence on dislocation density in neighbouring grains. To
allow modelling of thicker films, we propose a discrete dislocation model of
diffusional creep to investigate the effect of PG on the flow stress of submicron
films. A deformation map summarizes the range of dominance of different
strain relaxation mechanisms in ultra-thin films. We show that besides the
classical ‘threading dislocation’ regime, there are numerous novel mechanisms
once the film thickness approaches the nanoscale.